In the last decade, antennas constructed using printed circuit techniques have become popular, especially for mobile applications. These antennas are often thin and can be affixed to a vehicle, aircraft, etc. without appreciably altering the aerodynamics of the host structure.
In the prior art, a single slot has been used to enable printed circuit antennas to transmit and receive electromagnetic waves of circular polarization (CP). Kerr and others have demonstrated that strategically placing an elongated slot in the planar conductor of a printed circuit antenna can successfully enable the antenna to receive and transmit circularly polarized electromagnetic waves over a narrow band of frequencies (see "Microstrip Polarization Techniques", by John L. Kerr in the Proceedings of the 1978 Antenna Applications Symposium).
Later, it was shown that a single slot in the ground plane beneath a planar conductor of a printed circuit antenna can be used as both the method of excitation and the means to enable the antenna to operate in a circularly polarized (CP) manner (see "On slot-coupled microstrip antennas and their applications to cp operation--theory and experiment", by Aksum, Chuang, and Lo in the IEEE transactions on Antennas and Propagation, August 1990). However, the successful operation of either of the above-noted approaches is confined to an extremely narrow range of frequencies.
Greater CP bandwidths can be obtained from printed circuit antennas by providing multiple feeds. These feeds may be in the form of coupling apertures in the ground plane, probes extending between the planar antenna conductor and the ground plane, or microstrip transmission lines on the same surface as the antenna conductor.
In order to produce CP, proper amplitude and phasing of the exciting electromagnetic energy must be provided at the aperture feed points. The aperture feed systems of the prior art require power splitters and phase shifters, usually in the form of a 90-degree hybrid (see "Investigation of some antenna elements for advanced phased arrays", which is a Master of Science Thesis by A. Adrian, University of Mass., 1987). Although a large CP bandwith is thus obtained, the aperture-feed circuitry requires several components and occupies space, which may be in short supply.
Aperture-coupling techniques have several advantages over other feed methods. For example, layered construction using aperture coupling is applicable to microwave monolithic integrated circuit (MMIC) technology and eliminates the need for conductive interconnects between conductive layers. In addition, the multi-layer design allows the feed circuitry to be constructed on a material with a high dielectric constant, such as a semiconductor material, while the printed circuit antenna is placed above a substrate with a lower dielectric constant. The radiating element is thus, more efficient. Furthermore, the ground plane isolates the feed circuitry from the environment of the antenna, thus removing the effects due to radiation from the feed.
Accordingly, it is an object of this invention to provide a planar antenna which is excited by an aperture-coupling technique and employs a simple feed network.
It is a further object of this invention to provide a planar antenna which is adapted to transmit and receive waves of circular polarization.
It is another object of this invention to provide CP operation in a planar antenna over a greater bandwidth than prior art antennas employing single slot apertures.